A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein.
However, submersing the substrate or substrate and substrate table in a bath of liquid (see for example United States patent U.S. Pat. No. 4,509,852, hereby incorporated in its entirety by reference) means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors and turbulence in the liquid may lead to undesirable and unpredictable effects.
One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection system and the substrate (the substrate generally has a larger surface area than the final element of the projection system). One way which has been proposed to arrange for this is disclosed in PCT patent application WO 99/49504, hereby incorporated in its entirety by reference. As illustrated in FIGS. 2 and 3, liquid is supplied by at least one inlet IN onto the substrate, preferably along the direction of movement of the substrate relative to the final element, and is removed by at least one outlet OUT after having passed under the projection system. That is, as the substrate is scanned beneath the element in a −X direction, liquid is supplied at the +X side of the element and taken up at the −X side. FIG. 2 shows the arrangement schematically in which liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. In the illustration of FIG. 2 the liquid is supplied along the direction of movement of the substrate relative to the final element, though this does not need to be the case. Various orientations and numbers of in- and out-lets positioned around the final element are possible, one example is illustrated in FIG. 3 in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the final element.
A conventional lithographic projection apparatus requires one or more sensors on the substrate table so that, for example, the substrate table which carries the substrate can be correctly positioned relative to the projection beam. These sensors typically include a Transmission Image Sensor (TIS) which is a sensor that is used to measure the position at substrate level of a projected aerial image of a mark pattern at the reticle level (mask). Typically, the projected image at substrate level is a line pattern with a line width similar to projection beam wavelength. The TIS measures these mask patterns by using a transmission pattern with a radiation sensor underneath. The sensor data is used to measure the position of the mask with respect to the position of the substrate table in six degrees of freedom. Also the magnification and scaling of the projected mask pattern are measured, since four points on the mask are used for the measurement. As the sensor should also be capable of measuring the pattern positions and influences of all illumination settings (sigma, projection system NA, all masks (binary, PSM, . . . )), a small line width is required. Furthermore, the sensor is also used to measure/monitor the optical performance of the apparatus. Different measurements are implemented for measuring pupil shapes, coma, spherical aberration, astigmatism and field curvature. For these measurements, different illumination settings are used in combination with different projected images. Also such a sensor may be an Integrated Lens Interferometer At Scanner (ILIAS) which is an interferometric wavefront measurement system implemented on lithography tools. ILIAS performs (static) measurements on lens aberrations (up to Zernicke 36) as are needed for system setup and qualification. ILIAS is an on scanner integrated measurement system used for system setup and calibration. ILIAS is used for monitoring and recalibration of the scanner on a regular basis depending on the machine needs. Also, such a sensor may be a dose (spot) sensor or any other type of sensor that may be used at substrate level. All of these sensors are used at substrate level and as such are positioned on the substrate table. In order to avoid the need to perform complex predictions about how the immersion liquid will affect the projection beam, it is desirable to illuminate the one or more sensors under the same conditions as the substrate is to be imaged, i.e. with immersion liquid in place between the projection system and the sensor.
Sensors of the type mentioned above used in conventional lithographic projection apparatus typically have an absorption layer positioned over a grating in front of the actual radiation sensor. The absorbing layer is used to ensure that the sensor is a high contrast sensor so that accurate readings can be made. The absorbing layer has open and closed areas to get a high signal contrast between the radiation transmitted through the open patterns and the closed absorbing area. The photo sensor below the absorbing layer is normally much larger than the open patterns in order to measure the radiation for large angles. The ratio of the open pattern area versus the radiation sensitive area in a typical sensor is roughly (1:5600). So it is often important to absorb as much radiation as possible using an absorbing layer on the closed areas above the radiation sensor. Area patterns with a line width of the order of 200 nm are used. To implement this, a multi-layer structure is used so that the required resolution can easily be achieved. Typically the absorption elements of such sensors are made of a plurality of layers of different metal types. Chromium is the most widely used because it is common in mask production, and has good absorbing properties for blocking ultra-violet and deep ultra-violet radiation. Aluminum is also used because it has good etch selectivity with respect to chromium and a good optical density. Other metals, both elemental and alloys, may be suitable. Metals are typically used because of their good electrical conductivity and optical reflectivity which is useful for substrate table height measurements.
In United States patent U.S. Pat. No. 5,825,043, sensors are arranged above the substrate table relying on reflection of radiation off the surface of the substrate table to avoid making sensors resistant to liquid. However, this may result in a loss in accuracy.